The recirculation of exhaust gases is a technology which can be used for a very wide variety of purposes in gas turbines. Thus, exhaust gas recirculation is proposed for the reduction of NOx emissions or for a reduction of the exhaust gas which is to be discharged, for example. During the recirculation of exhaust gases in a gas turbine, a significant proportion of the exhaust gas is tapped from the overall exhaust gas flow and after cooling and scrubbing can be fed to the intake mass flow of the gas turbine or to the compressor, wherein the recirculated exhaust gas flow is mixed with fresh air, and this mixture is then fed to the compressor.
As a result of exhaust gas recirculation, the carbon dioxide partial pressure in the exhaust gases can be increased to reduce the power losses and efficiency losses of power plants with carbon dioxide separation. Furthermore, exhaust gas recirculation has been proposed with the aim of reducing the oxygen concentration in the intake gases of gas turbines in order to reduce the NOx emissions as a result.
For exhaust gas recirculation, U.S. Pat. No. 7,536,252 B1, for example, describes a method for controlling an exhaust gas recirculation flow of a turbomachine which is fed back to the intake of the turbomachine via an exhaust gas recirculation system. In this method, a nominal exhaust gas recirculation proportion is determined, wherein the exhaust gas recirculation proportion is defined as the proportion of the exhaust gas flow to the intake flow of the turbomachine, and the actual value is adjusted to the nominal value.
A method for operating a gas turbine with exhaust gas recirculation is known from US2009/0145126, in which the exhaust gas composition is determined and a control element allows for a control of the exhaust gas recirculation in dependence upon the measured exhaust gas composition. A control of this type can lead to unstable controlling, especially during start-up or operation with fast transients in the low load range since the exhaust gas composition depends heavily upon the proportion of recirculated exhaust gases. In addition, the exhaust gas composition, or the permissible composition of the intake gases of the gas turbine, is heavily dependent upon the operating point of the gas turbine so that a reliable operation of the gas turbine with exhaust gas recirculation becomes more difficult for specific operating states.
In order to ensure that no combustible or explosive fuel-air mixtures are left behind in the recirculation line, it has been proposed to purge the recirculation lines before starting a gas turbine, for which additional valves and lines are used. For example, a purge system for purging an exhaust gas recirculation system is known from EP2060772.